fix bug in build script

Get the date from the passed revision.

build.sh

@@ -6,7 +6,7 @@ if [[ $# -ne 1 ]]; then
 fi
 
 revision="$1"
-commit_date=$(git show -s --format=%ci $CI_COMMIT_SHA | cut -c1-10)
+commit_date=$(git show -s --format=%ci ${revision} | cut -c1-10)
 
 asciidoctor-pdf -a revnumber=${revision} -a revdate=${commit_date} -o angelguide_en.pdf guide.adoc
 asciidoctor-pdf -a revnumber=${revision} -a revdate=${commit_date} -o angelguide_de.pdf -a lang=de guide.adoc